The United States has deployed 45 troops to protect US personnel and assets in South Sudan, amid intensifying fighting between rebels and government forces, the White House said Thursday.

In a letter to Congress, President Barack Obama said the force was sent Wednesday and "will remain in South Sudan until the security situation becomes such that it is no longer needed."

"Although equipped for combat, this force was deployed for the purpose of protecting US citizens and property," he added.

"This action has been directed consistent with my responsibility to protect US citizens both at home and abroad, and in furtherance of US national security and foreign policy interests, pursuant to my constitutional authority to conduct US foreign relations and as commander in chief and chief executive."

Obama said the force consisted of "approximately" 45 troops, without giving a precise number.

The growing violence has prompted fears the world's youngest nation could slide toward civil war.

The move came after the United States ordered all non-emergency embassy staff on Tuesday to leave South Sudan and stressed the onus was on the country's leaders to end the violence.

The US mission in the capital Juba is also due to suspend normal operations for the time being.

The fighting in the world's youngest nation has set off alarm bells in the international community, with the United Nations urging the warring groups to refrain from ethnic violence.

In an example of the danger facing foreign troops in the volatile country, three Indian peacekeepers were killed Thursday in an attack by ethnic Nuer youths on a United Nations base in Jonglei state. Other casualties are feared.

US authorities also urged Americans against travel to the troubled country, urging that "US citizens currently in South Sudan depart immediately."

Those who choose to stay in South Sudan "should review their personal security situation and seriously reconsider their plans to remain," it added.